1. Field of the Invention
The present invention relates to an automatic matching device provided in a waveguide, and more particularly, to an automatic matching device used for transmitting a microwave and reducing power losses of the microwave, which is to be transmitted to a load, by attenuating a standing wave developed in the waveguide to match the impedance of the waveguide to that of the load.
2. Description of the Related Art
It is known to provide an automatic matching device in a waveguide, which transmits a microwave generated by a magnetron to a load, in order to efficiently transmit the power of the microwave to the load by matching the impedance of the waveguide to the load. This automatic matching device efficiently transmits the power of the microwave to the load by detecting a standing wave developed in the waveguide and by automatically operating to attenuate this standing wave. In order to increase the efficiency of transmission of the microwave power to a much greater extent, the speed of automatic impedance matching operations must be increased, and the automatic matching device must be compact.
The automatic matching device includes a detector section for detecting the standing wave developed in the waveguide; a matching section for attenuating the standing wave by matching the impedance of the waveguide to that of the load; and a control section for operating the matching device in accordance with the signal received from the detector section to attenuate the standing wave.
As known matching devices, there are stub matching devices, 4-E matching devices, and E-H matching devices.
The stub matching device has two or three stubs inserted into a waveguide, and impedance matching of the waveguide to the load is performed by adjusting the lengths of the inserted portions of the stubs.
In an automatic matching device that employs such a stub matching device, the control section adjusts the lengths of the inserted portions of the stubs in accordance with the signal received from the detector section so as to match the impedance of the waveguide to that of the load.
As shown in FIGS. 1A and 1B, the 4-E matching device comprises a waveguide 1 having a rectangular cross section and four E-plane branch waveguides 3a to 3d connected to a wider side, or an E-plane 2, of the waveguide 1. Provided that one wavelength of the microwave that travels along the waveguide 1 is .lambda.g, the distance between the waveguides 3a, 3b and that between 3c, 3d is set to .lambda.g/4, and the distance between the waveguides 3b, 3c is set to 3.lambda.g/8.
A short-circuiting plunger 4 (hereinafter "short plunger") is provided in each of the waveguides 3a to 3d, and the impedance of the waveguide 1 is matched to that of the load by adjusting the position of the short plunger 4 within each of the waveguides 3a to 3d.
In an automatic matching device that employs such an 4-E matching device, the control section adjusts the positions of the short plungers 4 in accordance with the signal received from the detector section so as to match the impedance of the waveguide 1 to that of the load.
As shown in FIG. 2, the conventional E-H matching device comprises the waveguide 1, an E-plane branch waveguide 5 connected to the E-plane 2 of the waveguide 1, and an H-plane branch waveguide 6 connected to an H-plane, or a narrower side of the waveguide 1. As shown in FIG. 3, a short plunger 7 is provided in each of the waveguides 5 and 6, and impedance matching of the waveguide 1 to the load is performed by adjusting the positions of the short plungers 7 in the waveguides 5 and 6 within the range of .lambda.g/2.
In an automatic matching device that employs such an E-H matching device, the control section adjusts the positions of the short plungers 7 in accordance with the signal received from the detector section so as to match the impedance of the waveguide 1 to that of the load.
The detector section detects a standing wave developed in the waveguide and outputs the result of such detection to the control section. The detector section comprises three or more detecting diodes provided along the axis of the waveguide such that the tip ends of the detecting diodes are exposed to the interior of the waveguide. Output voltages from the diodes are fed to the control section as a power-distribution signal. In accordance with this power-distribution signal, the control section detects the presence/absence of a standing wave and controls the matching device so as to attenuate the standing wave or to match the impedance of the waveguide to that of the load.
Each of the detecting diodes has a varying input-power-to-output-voltage characteristic. The input-power-to-output-voltage characteristic of the detecting diode comprises a linear region, a square-curve region, and a saturation region, in which the output voltage changes very little with respect to a variation in the input power.
If a dynamic range of the power distribution within the waveguide exceeds the liner region and reaches the square-curve region of the detecting diode, for example, the output characteristic of the diode in the square-curve region must be corrected such that it becomes the same as the output characteristic in the linear region. To this end, the output voltage of each detecting diode is corrected by an analog circuit for characteristic correction purposes, and the thus-corrected output voltage is provided to the control section.
Further, if the dynamic range of the power distribution within the waveguide increases and reaches the saturation region of the detecting diode, the analog circuit corrects the output characteristic of the diode in the saturation range such that it becomes the same as the output characteristic in the linear region. The thus-corrected voltage is output. With such a circuit configuration, even if the input power of the detecting diode increases with the result that characteristic range of the detecting diode shifts to a different range, an error in the output voltage is corrected by the analog circuit, and the thus-corrected voltage is output to the control section.
In the stub matching device of the foregoing automatic matching devices, if the power of the microwave transmitted to the load by the waveguide is increased, an electric discharge is likely to occur between the tip end of the stub and the interior surface of the waveguide. Further, if the amount of insertion of the stub is increased in order to sufficiently match the impedance of the waveguide to that of the load, an electric discharge is likely to occur between the tip end of the stub and the interior surface of the waveguide.
As a result, it is difficult to ascertain the amount of insertion of the stub in order to sufficiently guarantee the range of impedance matching and to transmit a microwave having sufficient power to the load. Accordingly, it is difficult for the stub matching device to perform impedance matching with regard to a microwave having high power. More specifically, in the case of impedance matching with regard to a microwave of 2.45 GHz, impedance matching with regard to about 2 kW is the limit of the stub matching device.
The 4-E matching device is superior to the stub matching device in terms of resistance to power. However, as shown in FIG. 1A, the four E-plane branch waveguides 3a to 3d must be provided at predetermined intervals on the E-plane of the waveguide 1. Accordingly, if the length L of the matching device is increased, the size of a three-dimensional circuit constituting the matching device is increased accordingly.
Like the 4-E matching device, the E-H matching device is superior to the stub matching device in terms of resistance to power. However, as shown in FIG. 2, the E-plane branch waveguide protrudes from the plane E in the vertical direction, whereas the H-plane branch waveguide protrudes from the plane H in the horizontal direction, which causes the three-dimensional circuit constituting the matching device to be bulky.
Further, in the E-H matching device, an unwanted high-order mode with respect to the frequency .lambda.g of the microwave to be transmitted is likely to be generated due to the presence of the H-plane branch waveguide, so that the power distribution of the standing wave is susceptible to disturbance. As shown in FIG. 16, at the time of a matching 25 operation, when an attempt is made to move a normalized resistance R to a matching point P along a circle R=1 by moving the short plunger provided in the E-plane branch waveguide or to move a normalized conductance G to the matching point P along a circle G=1 by moving the short plunger provided in the H-plane branch waveguide, neither the normalized resistance R nor the normalized conductance G moves along the corresponding circle. As a result, it becomes considerably difficult for the automatic matching device, which employs such an E-H matching device, to perform automatic matching operations.
Another problem suffered by the E-H matching device is that when the detector section detects the standing wave by detecting the distribution of power in the waveguide, owing to the disturbance of the power distribution, it becomes impossible for the detector section to accurately detect the standing wave. In order to reduce the influence of the E-H matching device exerted on the detector section, the distance between the E-H matching device and the detector section must be increased. However, such a circuit configuration results in an increase in the size of the three-dimensional circuit constituting the matching device.
The detector section must correct the characteristic variation of each detecting diode by using an analog circuit. If the dynamic range of the power distribution in the waveguide extends from the linear region to the square curve region of the detecting diode, the output characteristic variation of the diode must be corrected by the analog circuit, and the power distribution must be detected in accordance with the corrected output voltage of the detecting diode. Further, if the dynamic range of the power distribution in the waveguide reaches the saturation region of the detecting diode, a corresponding output voltage variation must also be corrected by the analog circuit.
As a result, the analog circuit becomes complicated, and the adjustment of the output characteristic of the circuit becomes considerably complicated. Moreover, it is impossible to completely correct all the variations in the characteristics. Accordingly, the distribution of power in the waveguide, or the standing wave, cannot be accurately detected.
If the detecting diode must be exchanged with a new one, the analog circuit must be readjusted in accordance with the output characteristic of the new detecting diode. In contrast, if the analog circuit must be replaced with a new one, the output characteristic of the new analog circuit must be adjusted in accordance with the output characteristics of the detecting diodes to be connected to the analog circuit. As described above, replacement of the detecting diode or the analog circuit requires very complicated exchange operations.